1. Field of the Invention
The present invention relates to a starter of a flat commutator type for starting a vehicle engine or the like.
2. Prior Art
FIG. 1 shows a conventional starter of a flat commutator type, which comprises the armature 1 of a DC motor; a rotary shaft 2 of the armature; flat commutator 3 fitted behind the armature and connected to an armature coil 1a; brushes 4 provided in a rear cover 5 made of plastic and pushed onto the surface of the flat commutator 3 by brush springs 6; a bearing 7 secured to the rear cover 5 and supporting the rotary shaft 2; fixed contacts 8 of an electromagnetic switch 9 which are attached to the rear cover 5 by nuts 10; movable contacts 11 of the electromagnetic switch which are attached with interposed electric insulators 12 to a plunger rod 13; a core 14; a driving coil 15 wound on a plastic-made bobbin 17 and secured to a case 18 to drive a plunger 16; an internal opening 19 provided in the plunger; a rod 21 urged forward by a spring 20; a yoke 22 of the DC motor; a permanent magnet 23 provided on the inside circumferential surface of the yoke to act as the field of the DC motor; a front cover 25 secured to the end face of the yoke 22 and formed with an internal gear 24 located on the inside circumferential surface of the rear portion of the front cover and constituting a planetary speed reducer; a spur gear 26 formed as the sun gear of the planetary speed reducer at the front end of the rotary shaft 2 of the armature 1 and engaged with a planet gear 27; a bearing 28 fitted on the inside circumferential surface of the planet gear and supported by a support pin 30 secured to the rear portion of an overrunning clutch 29 composed of an outer member 29a, an inner member 29b and rollers 29c located therebetween; a bearing 30 provided between the inner member 29b of the overrunning clutch 29 and a recess 25a of a front cover 25 and carrying as radial load; a rotary output shaft 31 which receives the action of the plunger 16 through the rod 21 and a steel ball 32 and is fitted to the inside surface of the inner member 29b of the overrunning clutch 29 so that the rotary output shaft is slidable back and forth; a sleeve bearing 33 supporting the rotary output shaft 31 at the rear end thereof; a bearing 34 supporting the rotary shaft 2 of the armature 1 at the front portion of the rotary shaft and fitted on the inside surface of the end portion of the yoke 22; and a pinion 35 which is fitted on a straight spline 31a formed on the outside circumferential surface of the rotary output shaft 31 and is engaged with a stopper 36 so as not to move forward.
The operation of the conventional starter of the flat commutator type is now described. When an ignition switch not shown in the drawing is turned on, electricity is applied to the driving coil 15 of the electromagnetic switch 9 to move the plunger 16 forward to put the movable contacts 11 into contact with the fixed contacts 8. As a result, electricity is applied to the armature coil 1a through the fixed contacts 8, lead wires not shown in the drawings which connect the contacts 8 and the brushes 4, the brushes 4 and the flat commutator 3 so that a turning force acts to the armature 1. In the meantime, the motive power of the moved plunger 16 is transmitted to the rotary output shaft 31 through the rod 21 to move the rotary output shaft forward so that the pinion 35 fitted on the rotary output shaft 31 is engaged with a ring gear for an engine not shown in the drawing. As a result, the engine is started.
However, since the flat commutator 3 is always pushed forward by the brushes 4 urged by the brush springs 6, there is a problem that the rotary shaft 2 of the armature 1 always receives a forward force to cause a torque loss at a thrust bearing.